Apparatus and method for improved power flow control in a high voltage network

ABSTRACT

An apparatus for controlling the power flow in a high voltage network. A phase shifting transformer includes a tap changer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the national phase under 35 U.S.C. §371 ofPCT/SE2004/001241.

TECHNICAL FIELD

The present invention concerns an apparatus and a method for controllingthe power flow in an ac transmission system. More precisely theinvention concerns a control apparatus comprising a phase shiftingtransformer (PST). By a phase shifting transformer should in thiscontext be understood to include a single cored as well as a multiplecored transformer, both of which may comprise a symmetric or anasymmetric design. The phase shifting transformer may also compriseadditional voltage regulating means.

BACKGROUND OF THE INVENTION

A phase shifting transformer is previously known for controlling thepower flow in an ac transmission line. Such PST comprises a tap changerthat serially connect or disconnect additional windings of thetransformer. By doing so the phasor orientation is controlled. Power isthen moved from adjacent phases to a single phase by connections betweenwindings excited by different parts of the magnetic circuit. In a purephase shifting transformer a voltage in quadrature to the source voltageis injected into the line.

A phase shifting transformer may be used to control the loaddistribution between parallel lines to increase total power transfer.Advantageous is the phase shifting transformers capability to blockparasitic power flow due to phase angle difference in a feeding network.

Power may be distributed to customer in a defined way and circulatingpower flows may be avoided.

The use of a PST is advantageous in that it has a relatively lowreactive power consumption. There is no risk of a subsynchronousresonance (SSR) and it is powerful also at low current conditions.

The use of a PST however offers a slow control speed. The tap changerhas to go through every tap position in a sequential manner. Each tapchange is effected in the order of 3-5 seconds. Thus the PST cannotparticipate in a decisive way in a transient period following a powerdisturbance. Further frequent tap changing, in particular at highcurrent conditions, increases the need for maintenance.

The tap changer is a mechanical device and thus slow and an object tomechanical wear. It has a maximum regulation voltage range of 150 kV anda maximum number of operating positions of less than 35. The maximum tapvoltage is in the order of 4000-5000V/tap and the maximum rated throughcurrent is about 3000-4500 A. The maximum power handling capacity is6000-8000 kVA/tap and there is a short circuit thermal limit. Smallvoltage steps make many operations.

Another way to control the power flow in an ac transmission line is theuse of a controlled series compensator (CSC). Such CSC comprises one ora plurality of thyristor switched inductive devices. The CSC may alsocomprise one or a plurality of thyristor switched capacitive devices,often in combination with an inductor. The capacitive device or theinductive device is connected in a parallel branch with a thyristorswitch. By controlling the thyristor switch the inductive or thecapacitive device is connected or disconnected to the transmission line.Thus the phasor orientation is controlled by connecting or disconnectinga desired number or combination of inductances or capacitances. Theregulation is rapid since there is no mechanical devices involved.

A CSC is controllable from full inductive to full capacitive regulation,and vice versa, within a few fundamental frequency cycles and is thuscapable of being a powerful control device in the transient periodfollowing a power disturbance. In comparison with a tap changer of a PSTthe CSC is not maintenance sensitive to frequent control actions. A CSCis therefore suitable for closed loop control.

However in circuit comprising a CSC with capacitive steps there is arisk for resonance problems such as SSR. The CSC has a larger reactivepower consumption with large inductive steps in comparison to a PST. Atlow current conditions the CSC has a small impact on the power flow.

SUMMARY OF THE INVENTION

A primary object of the present invention is to provide a power flowcontrol of an ac power transmission that is rapid and that does notinvolve the drawbacks of the single use of either a PST or a CSC.

This object is achieved according to the invention by a controlapparatus or by a method.

According to the invention a PST containing a tap changer, a CSCcontaining an inductor, and a control unit that controls both the PSTand the CSC are combined to form a control apparatus for controlling thepower flow in a high voltage network. As a response to a change in theload conditions the power flow is controlled by firstly regulating theCSC and secondly regulating the PST by the tap changer. In a firstperiod of time the control is effected solely by the CSC device and in asecond period of time the control is effected by a combined regulationof both the CSC device and the PST. By this control the slow controlcapability of the PST is compensated for by the rapid capability of theCSC. For every change from one tap to another of the tap changer the CSCis controlled to compensate for the new tap position. Thus a favorableworking condition of the apparatus may be achieved within thecontrolling ranges of the PST and the CSC. In a first embodiment of theinvention the CSC comprises a thyristor switched inductor function. In afurther embodiment of the invention the CSC comprises a thyristorswitched capacitor function which may be combined with an inductorfunction.

In a first aspect of the invention the object is achieved by a controlapparatus comprising a PST including a tap changer, a CSC including acontrollable inductive means, and a control system containing computermeans including a processor for controlling the PST and the CSC incoordination. In a preferred embodiment the CSC also contains acontrollable capacitive means. In a further embodiment the controllableinductive means comprises a plurality of inductive units, eachcomprising an inductor in a parallel connection with a thyristor switch.In yet a further embodiment the capacitive means comprises a pluralityof capacitive units, each comprising a capacitor in parallel connectionwith a thyristor switch. In yet a further embodiment the capacitive unitcomprises an inductor in series with the thyristor switch. The controlsystem comprises in a further embodiment a communication unit by whichthe control is supervised, controlled or overridden by an operator or acustomer.

In a second aspect of the invention the objects are achieved by a methodfor controlling the power flow in an ac transmission line, the methodcomprising a first step in which the new load demand is rapidlyregulated by the CSC and a second step in which a combined regulation ofboth the PST and the CSC is evaluated. In a further step an internalregulation of a favorable working point for both the PST and the CSC isaccomplished by regulating the PST in coordination with the CSC suchthat the external control is unaffected.

The PST which is regulated in sequential steps by the tap changer isaccording to the invention combined with a CSC which is regulated bythyristor switches, to provide a fast and adaptive control of the powerflow by a common control system. The slow control capability of the PSTis compensated for by the rapid control of the CSC. The PST is thusdynamically assisted by the CSC when regulating. This dynamicallyassisted PST, in the following text denoted DAPST, comprises a standardtap changer controlled phase-shifting transformer combined withthyristor switched inductive and/or capacitive reactance circuits. Eachsuch circuit may comprise a plurality of inductive and capacitivecircuits which may be connected in steps. The dynamic assistance of thePST reduces the number of control actions made by the tap changer, whichdramatically increases the life time cycle of the tap changer.

According to the invention the required rating of a power flowcontroller (PFC) is divided into two parts, one part consisting of thePST and the other part consisting of the CSC containing thyristorswitched reactance circuits. The possibility to coordinate the controlof the CSC and the PST offers the rating of both units to be smallerthan in circuits where each unit works alone. As a comparison a singlePST regulating unit would have to have a large rating and a single CSCunit would have to have an increased number of reactance circuits. Thecombination will obtain an overall improvement of the performance ascompared to both the PST and the thyristor switched reactance stepsonly.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the present invention will become moreapparent to a person skilled in the art from the following detaileddescription in conjunction with the appended drawings in which:

FIG. 1 is a principal circuit of a control apparatus according theinvention,

FIG. 2 is a one sided discrete function of the apparatus,

FIG. 3 is a discrete control function of the apparatus,

FIG. 4 is a continuous control function of the apparatus,

FIG. 5 is a simple network comprising the apparatus,

FIG. 6 is the operating range in terms of series voltage and throughputcurrent,

FIG. 7 is the control apparatus including a CSC-part comprising threeinductive thyristor switched units,

FIG. 8: is the operating range in purely inductive CSC mode,

FIG. 9: is the operating range in purely PST mode,

FIG. 10: is the steady state operating range of the apparatus,

FIG. 11: is the dynamic range of the apparatus with PST in maximum tapposition,

FIG. 12: is the dynamic range with PST in minimum tap position,

FIG. 13: is an apparatus combined with shunt compensation means.

FIG. 14 is a conceptual control scheme of the apparatus, and

FIG. 15 is an apparatus with a control unit that includes a computerincluding a processor and a memory unit and a sensor for sensing thepower flow of the network.

DESCRIPTION OF PREFERRED EMBODIMENTS

An apparatus for controlling the power flow according to the inventionis shown in FIG. 1. The apparatus comprises a tap changer controlledphase shifting transformer (PST) 1, a controlled series compensator(CSC) 2 and a control unit 3. The CSC comprises a first reactance unit 4that includes an inductive unit 6 and a thyristor switch 7 forconnecting and disconnecting the inductive unit. In the embodiment shownthe CSC 2 also comprises a second reactance unit 5 that includes acapacitive unit 8 as well as a inductive unit 6 that are controlled by athyristor switch 7. The single inductive and capacitive reactance unitsare shown by way of example. It lies within the scope of the inventionto combine any number of inductive and capacitive steps. Thus thecontrolled series compensation device may comprise a plurality of bothinductive and capacitive circuits. The control unit 3 may include acomputer 26 including a processor 27, a memory unit 28 and a sensor 29for sensing the power flow of the network, as shown in FIG. 15.

The CSC may be realized in different configurations. In a firstembodiment the CSC comprises switchable inductive units by which the CSCis controllable in discrete steps. In a second embodiment the CSCcomprises a combination of inductive and capacitive units and thus beingcontrollable in discrete steps. In a third embodiment the CSC comprisesa plurality of inductive step and a plurality of boostable capacitivesteps that offers the CSC to be continuously controllable.

In a one-sided discrete embodiment of the invention the CSC comprisesonly inductive units. In this embodiment there is thus no risk ofresonances (e.g. SSR). Assuming that the two inductive steps arethyristor switched and thatX_(L2)=2X_(L1)

The CSC units have thus an inductive control range divided into discretesteps as illustrated in the FIG. 2.

The control of the power flow by the PST is accomplished through controlof the tap-changer. Since this is a mechanic device and the control hasto be done in sequential steps this control is slow. For the CSC each ofthe four positions in FIG. 2 may be assumed rapidly.

In a further embodiment of the invention the CSC comprises a pluralityof both inductive and capacitive units. The discrete controllingcapability is illustrated in the following way. Assume that both thecapacitive step and the two inductive steps are thyristor switched (i.e.no boosting which produces harmonics), and thatX_(L2)=2X_(L1)|X _(L2) |=|X _(C)|

The CSC parts have thus an inductive and capacitive control rangedivided into discrete steps as illustrated in the FIG. 3.

A continuous controllable embodiment according to the invention isillustrated in the following way (the number of steps can of course bechanged) In this embodiment the CSC comprise a plurality of inductiveunits and a plurality of capacitor units that are continuouslycontrollable (boostable). Assume that the capacitive part is boostablesuch that it is continuous controllable between|X _(CB) ^(max) |≧X _(CB) ≧|X _(CB) ^(min)|where X _(CB) ^(max)=2X_(CB)^(min)and that the inductive part is divided into two parts, X_(L1) andX_(L2), where|X _(L1) |=|X _(CB) ^(min)|X_(L2)=2X_(L1)

The CSC parts are thus continuous controllable in the range illustratedin FIG. 4.

The control resolution of the apparatus according to the invention (thecombined effect of tap changer and CSC control action) is in thisembodiment made infinite over a large portion of the combined controlrange.

In a further advanced embodiment of the invention means for reactivepower shunt compensation is included, as indicated in FIG. 13, such thatthe reactive power balance of the DAPST may be customized to meet the aparticular power system requirements. These means include devices suchas for instance circuit breaker connected capacitor banks and reactors,static var compensators (SVC) and STATCOM.

In order to illustrate the operating range of a DAPST a simple networkas shown in FIG. 5 is used. The network comprises a DAPST according tothe invention in a parallel connection with an equivalent reactancebetween a first node 14 and a second node 15. The purpose of a DAPST isto control the distribution of power between on one hand the path inwhich it is installed and on the other hand parallel paths. The parallelpaths are in FIG. 5 represented by the equivalent inductive reactanceX_(eq) connected in parallel to the DAPST. By controlling the seriesvoltage V_(Series) the distribution of power flow can be controlled.

The bold faced quantities in FIG. 5 represent phasors (with both amagnitude and a phase). Currents are represented with I and voltageswith V.

With this simple network, the operating range may be described in adiagram as indicated in FIG. 6, where the current through the DAPST ison the x-axis and the series voltage is on the y-axis.

The left half plane corresponds to current (power) flowing from thesecond node 15 to the first node 14 (called import) and the right halfplane corresponds to current (power) flowing from the first node to thesecond node (called export). The first and third quadrant correspond toa reduction of the magnitude of the current (power) whereas the secondand fourth correspond to an increase of the magnitude of the current(power).

A DAPST comprising a PST 1 and a CSC including a plurality of switchedinductive units 4 a-4 c as indicated in FIG. 7, it may be operated in apurely Controllable Series Compensator (CSC) mode with thePhase-Shifting Transformer (PST) at zero tap position (which impliesthat only the short circuit reactance of the PST contributes to theseries voltage.)

FIG. 8 illustrates the operating range in purely inductive CSC mode.Along the bypass line all CSC steps are thyristor by-passed. The slopeof the line depends on the short circuit reactance of the PST.

Starting from the by-pass line, the voltage across the CSC is increasedand the current through the DAPST is reduced by switching in inductivesteps. The operating point will thus move along lines parallel to thearrows in FIG. 8 (the slopes of the arrows will depend on the size ofX_(eq)). By giving the inductive steps different sizes (e.g. binarysizes) and adding additional steps the resolution of control can be madearbitrarily high. The CSC will typically be dimensioned such thatoperation is only allowed for series voltages across the individualsteps below limits that are predetermined. When all inductive steps areswitched in, the series voltage will decrease if the current decreasesfurther. It can be noted that for low currents, the CSC is quitepowerless as hardly any series voltage can be provided even if verylarge inductive steps were available. In a similar fashion, capacitivesteps would give an operational range in the second and forth quadrants.

With the CSC part thyristor by-passed (both inductive and possiblycapacitive steps) the DAPST may be operated in a pure PST mode. Theoperating range may then look as indicated in FIG. 9.

The zero tap line is the same as the by-pass line in FIG. 8, i.e. itsslope depends on the short circuit reactance of the PST. Starting fromthe zero tap line, the series voltage increases in the positivedirection when the tap-changer is moved towards the most positiveposition and the magnitude of the series voltage increases in thenegative direction when the tap-changer is moved towards the mostnegative position. It can be noticed that this PST has the ability toboth decrease the power flow (first and third quadrant operation) andincrease the power flow (second and fourth quadrant operation) ascompared to the zero tap line. Furthermore, the PST has a substantialcapability to control the power flow also at low current conditions.

FIG. 10 illustrates the DAPST steady state control range when both thePST and the inductive CSC ranges are combined.

With the choice of inductive CSC steps, the operating range is extendedin the first and third quadrants (corresponding to reduction of power)as compared to the pure PST mode. With capacitive CSC steps it is in asimilar fashion possible to extend the operating range in the second andfourth quadrant.

With dynamic operating range it is meant the part of the operating rangethat can be controlled fast enough to mitigate the consequences ofelectromechanical transients in a power system. As the thyristorcontrolled CSC easily can change operating point from minimum inductive(or maximum capacitive if capacitive steps are available) to maximuminductive within a fraction of a second, it is well suited forcontributing to e.g. damping of power oscillations originating fromelectromechanical oscillations in synchronous machines (generators). ThePST on the other hand, where each step takes in the order of fiveseconds and each step must be sequentially passed, is too slow toactively contribute in the transient period. In other words, the dynamicpart of the operating range of he DAPST corresponds to the CSC part.

However, by controlling the tap-changer in the pre-disturbance situationthe over-all characteristics of the DAPST can be changed. FIG. 8 ofcourse gives the dynamic range with the PST in zero tap position.

In FIG. 11 the PST is in the maximum tap position. It can be noted thatin the import situation (negative throughput currents), the DAPST hasthe over-all dynamic characteristic of being capable of both increasingand decreasing the power flow dynamically (in other words, the lineV_(series)=0 passes through the dynamic operating range). Of course asimilar feature can be achieved in the export situation by setting thePST in e.g. the minimum tap position, as indicated in FIG. 12.

It is consequently possible to give the DAPST the possibility todynamically both increase and decrease the power flow. The circumstancethat this is achievable with an inductive CSC, i.e. without a seriescapacitor, is a major advantage in systems with thermal productionplants, e.g. nuclear power plants, with complex turbine strings. As iswell-known installation of series capacitors in such systems requiressubstantial analysis and adequate control means to avoid the risk ofsub-synchronous resonance which may seriously damage the productionunits.

In particular, if a DAPST with an inductive CSC also includes means forcapacitive shunt compensation, such as capacitor banks connected withcircuit breakers, as indicated in FIG. 13, the DAPST can be givencharacteristics very similar to a controllable series capacitor, also interms of reactive power balance, without the risk of e.g.sub-synchronous resonance.

The main control objectives, i.e. the reasons for installing a controlapparatus, which may be denoted a dynamically controllable phase shifttransformer (DAPST), include one or several of the following;

-   -   Slow/quasi-steady state power flow control    -   Power oscillation damping    -   Improving transient performance through fast change of        transmission corridor characteristics

Slow/quasi-steady state power flow control is the slow control of thedistribution of power between on one hand the transmission path in whichthe DAPST is installed and on the other hand parallel paths. The controlspeed requirements for meeting this objective are low enough to besatisfied by both the PST and the CSC parts.

Power oscillation damping is the fast control of the DAPST to mitigatepower oscillations typically following a disturbance in the powersystem. The frequency of these oscillations are typically in the rangeof 0.1-2.0 Hz and depends to a large extent on the inertia constants ofthe synchronous machines (typically generators) or groups of machinesparticipating in the oscillation. The control speed requirement to meetthis objective can only be satisfied by the CSC part of the DAPST.

By quickly, within a fraction of the transient period following adisturbance, changing the operating point of the CSC part of the DAPST,the character of the transmission interconnection on which the DAPST isinstalled can be changed. In its most inductive position the totalreactance of the transmission interconnection is at its maximumresulting in a reduced power transfer over the interconnection and anincreased power transfer over parallel paths. In its most capacitive (orleast inductive) position the total reactance of the interconnection isat its minimum resulting in an increased power transfer over theinterconnection and a reduced power transfer over parallel paths. Inparticular if several DAPST are installed and their controls arecoordinated, the disturbed part of the system experiencing e.g.stability problems can quickly be relieved of power transfer whereas anintact part of the system picks up the power transfer. By having thecapability to go between end positions within say less then 0.5 s afterthe disturbance occurs, the transient performance of the overallinter-connected system can thus be significantly improved. The PST partis too slow to act within this time frame however its pre-disturbanceoperating point will affect the overall character of the DAPST and thusthe transmission interconnection.

By coordinating the control of the tap-changer and the thyristors, it isalso possible to include one or several of the following advantageouscontrol objectives in the control strategy:

-   -   Operation of the tap-changer at lowest possible current    -   Smallest possible number of tap-changer operations    -   Reactive power consumption control (limitation)    -   Dynamic range control

By controlling the DAPST at high current conditions such that the CSCpart primarily acts first to reduce the current and then the PST partacts primarily at lower current conditions, the stresses on thetap-changer are relieved and the need for maintenance is reduced.

By letting the CSC part be as fast as possible, and slowing down the PSTpart even more than it is by nature, the number of tap-changeroperations can be reduced. This is accomplished as changes in loadingwith short duration are handled by the CSC part and the PST part onlyacts on changes of longer duration.

The PST part consumes reactive power due to its short circuit reactance,which only has a small variation due to tap-changer position as comparedto the CSC which reactance has a substantially larger variation due toits nature. The CSC may as a consequence consume or produce aconsiderable amount of reactive power if all inductive or capacitivesteps are switched in. As almost the whole operating range can bereached with different mixes of PST and CSC control action the overallconsumption or production of reactive power can be affected. If e.g. thepower system for some reason is weak in terms of voltage support, i.e.the voltages are low it is advantageous to produce reactive power or atleast limit the overall consumption of reactive power of the DAPST. Ifthe desired series voltage can be achieved through the combined actionof capacitive CSC steps (if available) and PST, as much CSC action andas little PST action is advantageous. If the desired series voltage onlycan be achieved through the combination of inductive CSC steps and PSTaction, as much PST action and as little CSC action is advantageous.Similarly, if the voltages are high it is advantageous to consumereactive power or at least limit the overall production of reactivepower of the DAPST. Obviously the opposite control strategy is to bepreferred, i.e. if the desired series voltage can be achieved throughthe combined action of inductive CSC and PST, as much CSC action and aslittle PST action is advantageous. If the desired series voltage onlycan be achieved through the combination of capacitive CSC steps (ifavailable) and PST action, as much PST action and as little CSC actionis advantageous. In addition it is possible to control the over allreactive power balance if means for reactive power shunt compensation isadded to the DAPST as mention above.

Yet another advantageous control objective is the control of dynamicrange at low current conditions. At low line currents the control rangeof the CSC part becomes small and even zero. By controlling the PST partsuch that there almost always is a minimum line current available, aminimum dynamic range is consequently also almost always available.(There may be a short time period with low current when the powerdirection is changed from import to export, but this time period can bemade very short by proper control of the tap-changer.)

A control scheme of a DASPT for controlling the power flow according tothe invention is shown in FIG. 14. In the embodiment shown the DAPSTcomprises PST and a CSC arranged on a power transmission line 10, afirst closed loop 11 and a second closed loop 12. The first closed loopcomprises a sensor 13 for sensing the power flow on the line 10 betweena first node 14 and a second node 15. Further the first loop comprises afirst comparator 16, a PI (Proportional-Integrate) controller 17 and asecond comparator 18.

The measured active power flow, P^(m), is compared with a set value,P^(set), corresponding to the desired active power flow. A differencesignal is sent to a PI controller with limits. The PI-controller createsa signal proportional to the required reactance which is sent to thethyristor control of the CSC, which by these means is controlled tosatisfy the set value of active power flow. The discrete nature of theCSC will in most situations result in a control error which is handledthrough dead-bands (not indicated in the drawing).

A supplementary signal for Power Oscillation Damping (POD) may be addedafter the PI controller such that fast electromechanical poweroscillations can be mitigated by CSC action.

The second closed loop for controlling the PST comprises a low pass 15filter 19, a comparator 20 and a PI-controller 21. The signalproportional to the required reactance is sent through the low-passfilter 19 for comparison to a set value of the reactance, {tilde over(x)}. The low-pass filter will block variations of in x of shortduration. The difference between the actual value of x and the desiredvalue {tilde over (x)} is sent to a PI 20 controller with limits whichcreates a signal proportional to the desired tap, t, which is sent tothe tap-changer control.

It is possible to achieve all control objectives listed above byapplication of the control scheme in FIG. 14.

-   -   Slow/quasi-steady state power flow control is obviously        achieved.    -   Power oscillation damping is achieved by introduction of the        supplementary POD signal.    -   Fast change of transmission corridor characteristics can be        achieved by quickly changing the set point P^(set), possibly        combined with changing the gain of the first PI-controller.    -   Operation of the tap-changer at high current conditions is        avoided in the following way. If the current suddenly increases,        in particular into the overload range, the CSC will first act to        reduce the current, and then at lower currents the PST will act        to satisfy the set value {tilde over (x)}.    -   The number of tap-changer operations is reduced by the        introduction of a low-pass filter in the tap-changer control        branch. This filter will block changes of short duration and        thus reduce the number of tap-changer operations.    -   Reactive power balance control (limitation) can be realized in        at least two ways. By changing the set value {tilde over (x)}        the reactive power balance can be controlled, and by introducing        reactive power limits in the first PI-controller limiter it can        be limited.    -   Control of the dynamic range at low current conditions can be        accomplished by control of the set value P^(set). By using a        limit p^(set,limit)<|P^(set)| a minimum magnitude of the line        current, and thus a minimum control range, can be realized in        steady state.

Other control schemes can of course also be used to meet the controlobjectives.

Although favorable the scope of the invention must not be limited by theembodiments presented but contain also embodiments obvious to a personskilled in the art. For instance the transmission line between the PSTand the SCS must not be short as indicated in the accompanied figuresbut comprise any length as only the PST and the SCS is seriallyconnected. The closed loop arrangement in FIG. 13 must not be fullyclosed. Thus it may under certain conditions be favorable for anoperator or a customer to choose a desirable working point by directadjustment of the tap changer of the PST. According to the invention theapparatus would automatically respond to such a forced control of thePST by adjusting the CSC correspondingly.

1. An apparatus for controlling a power flow in a high voltage network,comprising a phase shifting transformer, comprising a tap changer, acontrolled series compensator containing a controllable inductor unit,and a control unit configured to control the phase shifting transformerand the controlled series compensator in coordination.
 2. The apparatusaccording to claim 1, wherein the controllable inductor unit comprisesat least one unit comprising an inductor device in parallel connectionwith a thyristor switch.
 3. The apparatus according to claim 1, whereinthe controlled series compensator comprises a controllable capacitorunit.
 4. The apparatus according to claim 3, wherein the controllablecapacitor unit comprises at least one unit comprising a first branchcomprising a capacitive device and a second branch connected in parallelto the first branch comprising a thyristor switch.
 5. The apparatusaccording to claim 4, wherein the second branch comprises an inductordevice in series with the thyristor switch.
 6. The apparatus accordingto claim 3, wherein the controlled series compensator comprises aplurality of inductive units and a plurality of boostable capacitiveunits.
 7. The apparatus according to claim 1, wherein the control unitcomprises a first loop for controlling the controlled seriescompensator.
 8. The apparatus according to claim 1, wherein the controlunit comprises a second loop for controlling the phase shiftingtransformer.
 9. The apparatus according to claim 1, wherein the controlunit comprises an introducing unit configured to introduce controlparameter values.
 10. The apparatus according to claim 1, wherein thephase shifting transformer is positioned at a first location and thecontrolled series compensator is positioned at a second location andwherein the first and second location is separated by a distance. 11.The apparatus according to claim 1, further comprising: a shuntcompensator.
 12. The apparatus according to claim 11, wherein thecontrol unit comprises a shunt compensator configured to control theshunt compensator.
 13. The apparatus according to claim 1, wherein thecontrol unit comprises a computer including a processor and a memoryunit and a sensor for sensing the power flow of the network.
 14. Amethod for controlling a power flow in a high voltage network having acontrol apparatus including a phase shifting transformer, the methodcomprising: adjusting in a first period of time a reactance of thecontrol apparatus by a controlled series compensator, and adjusting in asecond period of time a voltage of the phase shifting transformer andthe reactance of the control apparatus to achieve a favorable workingbalance between the phase shifting transformer and the controlled seriescompensator.
 15. The method according to claim 14, wherein thecontrolled series compensator adjustment comprises sensing the powerflow, comparison with a set value, adjustment by a PI-controller andcomparison with a oscillation damping signal.
 16. The method accordingto claim 15, wherein the phase shifting transformer adjustment comprisesfiltering a control signal from the controlled series compensatoradjustment, comparison with a reactance set value and adjustment by aPI-controller.
 17. A computer program product, comprising: a computerreadable medium; and computer program instructions recorded on thecomputer readable medium and executable by a processor to carry out amethod comprising adjusting in a first period of time a reactance of acontrol apparatus by means of a controlled series compensator, andadjusting in a second period of time a voltage of a phase shiftingtransformer and the reactance of the control apparatus to achieve afavorable working balance between the phase shifting transformer and thecontrolled series compensator.
 18. The computer program productaccording to claim 17, wherein the computer program instructions arefurther for carrying out a method comprising providing the computerprogram instructions at least in part over a network.
 19. The computerreadable medium, according to claim 18, wherein the network comprisesthe internet.